The search for new alloys for aircraft parts satisfactory for severe operating conditions, particularly in engines, must take into account various parameters, including high temperature stability and a minimum mass for a specific level of mechanical characteristics. The use of niobium (Nb) could meet these conditions through a high melting point (2470.degree. C.) and a relatively low density (8.6).
EP-A-0 345 599 discloses a group of alloys of which the atomic percentage composition comprises from 31 to 48% Ti, from 8 to 21% Al, and Nb as the remainder. Some advantageous mechanical properties are exhibited by these known alloys. However, they are not entirely satisfactory because the concentration of aluminum is inadequate to permit the formation of a continuous layer which is sufficiently protective in an oxidizing medium at high temperatures for some of the particular applications, especially in the aeronautical field, which are envisaged by the present invention.
It is, in fact, generally known that niobium based alloys have a low resistance to high temperature oxidation because the Nb.sub.2 O.sub.5 oxide formed does not constitute a protective layer. One of the oxides which would possess such a property is aluminum oxide Al.sub.2 O.sub.3.
In addition to layer flaking problems, however, serious obstacles stand in the way of the development of alloys capable of forming a continuous coating of aluminum oxide. On the one hand, a minimum concentration of aluminum is necessary for the formation of the layer and, on the other hand, very high temperatures, above 1200.degree. C., are required for activation of the protective coating forming process. Indeed, the formation of a continuous coating of aluminum oxide appears impossible in the case of a binary Nb-Al alloy because a high aluminum content, although improving the resistance of the binary alloy to oxidation, is detrimental by virtue of the formation of fatigue-producing phases.
The addition of a third element, such as titanium, enables the quantity of brittle phases to be reduced and, at the same time, promotes the formation of a more protective layer of oxides.
For certain uses, the known alloys do not possess an adequate resistance to oxidation in the vicinity of 900.degree. C. The aforementioned EP-A-0 345 599 recommends making protective coatings to improve the resistance to oxidation of parts made from the alloys described. However, these techniques have their own drawbacks, such as increased costs and manufacturing steps, and the need for additional means to implement the process.